Belgian police raided the offices of Anderlecht and the national football association on Wednesday in relation to a money laundering investigation involving agents and player transfers - including the £13m sale of Aleksandar Mitrovic to Newcastle in 2015.
A spokeswoman for the Brussels club told public broadcaster VRT that Anderlecht was cooperating fully with the probe and a spokesman for the Belgian FA told Reuters that police had taken away documents relating to transfers from its headquarters.
A spokesman for the federal prosecutor said: 'We had some questions for the agents who took care of the transfers.
It is a question of money laundering.'
Newcastle did not immediately respond to a request for comment.
Mitrovic now plays for Fulham and joined the London club on permanent terms last summer.
VRT and other local media said prosecutors were looking into transfers made before a 2017 change of ownership at Anderlecht and had also raided an agent's office in the capital.
The prosecutor's spokesman declined to give details of the three raids in Brussels on Wednesday.
He said that they were not linked to the wide-ranging 'Footballgate' probe into match-fixing and fraud which rocked the domestic league last year after the national side's third place finish at the World Cup.